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Cardiac neural crest cells undergo extensive cell rearrangements during the formation of the aorticopulmonary septum in the outflow tract. However, the morphogenetic mechanisms involved in this fundamental process remain poorly understood. To determine the function of the Ca2+-dependent cell adhesion molecule, N-cadherin, in murine neural crest, we applied the Cre/loxP system and created mouse embryos genetically mosaic for N-cadherin. Specifically, deletion of N-cadherin in neural crest cells led to embryonic lethality with distinct cardiovascular defects. Neural crest cell migration and homing to the cardiac outflow tract niche were unaffected by loss of N-cadherin. However, N-cadherin-deficient neural crest cells were unable to undergo the normal morphogenetic changes associated with outflow tract remodeling, resulting in persistent truncus arteriosus in the majority of mutant embryos. Other mutant embryos initiated aorticopulmonary septum formation; however, the neural crest cells were unable to elongate and align properly along the midline and remained rounded with limited contact with their neighbors. Interestingly, rotation of the outflow tract was incomplete in these mutants suggesting that alignment of the channels is dependent on N-cadherin-generated cytoskeletal forces. A second cardiac phenotype was observed where loss of N-cadherin in the epicardium led to disruption of heterotypic cell interactions between the epicardium and myocardium resulting in a thinned ventricular myocardium. Thus, we conclude that in addition to its role in myocardial cell adhesion, N-cadherin is required for neural crest cell rearrangements critical for patterning of the cardiac outflow tract and in the maintenance of epicardial-myocardial cell interactions.

Pax3 is a transcription factor expressed in the dorsal neural tube and somite of the developing embryo. It plays critical roles in pre-migratory neural crest cells and in myogenic precursors of skeletal muscle. Pax3-deficient Splotch embryos display neural tube and neural crest defects and lack hypaxial muscles. We have created a new allele of Splotch by replacing the first coding exon with a gene encoding Cre recombinase. This functions as a null allele and no Pax3 protein is detected in homozygous embryos. Heterozygous Pax3(Cre/+) mice display a white belly spot, as do Splotch heterozygotes. Homozygous Pax3(Cre/Cre) embryos are embryonic lethal. We have used Pax3(Cre/+) mice to fate-map Pax3 derivatives in the developing mouse. As expected, neural crest and some somitic derivatives are identified. However, we also detect previously unappreciated derivatives of Pax3-expressing precursors in the colonic epithelium of the hindgut and within the urogenital system.